The present invention is generally applicable to communications systems. More particularly, the present invention is applicable to polarization control in optical communications systems that suffer from dead spot problems.
Communications systems employing optical equipment have long been used to provide high bandwidth transmission of vast amounts of data. Enhanced signaling techniques have been implemented in order to achieve even greater throughput, particularly for long distance (“long haul”) transmission. One important technique employs polarization. For an optical signal, polarization, or the state of polarization (SOP), represents the amplitude and direction of the electric field vector of the light signal.
By way of example only, an aggregate or composite optical signal may transmit numerous channels, each having a different wavelength and a different polarization. The channels can be polarized so that, e.g., all even channels are polarized to a first polarization state and all odd channels are polarized to a second polarization state. The first and second polarization states may be orthogonal to one another, substantially reducing unwanted cross talk between adjacent channels. Orthogonal polarization, also known as “orthogonal launch,” is more fully explained in U.S. Pat. No. 6,134,033, entitled “Method and Apparatus for Improving Spectral Efficiency in Wavelength Division Multiplexed Transmission Systems” and U.S. Pat. No. 6,459,515, entitled “Method and Apparatus for Transmitting a WDM Optical Signal Having States of Polarization That Are Pairwise Orthogonal,” the entire disclosures of which are fully incorporated by reference herein.
A critical issue when employing polarized signals is maintaining the SOP along the transmission path. For example, as signals are transmitted over optical fiber, the SOP may fluctuate based on a variety of factors, such as the type of fiber, the length of the fiber, manual handling, etc. Single-mode fibers, e.g., fibers that propagate only one mode above a cutoff wavelength, may not preserve the SOP of signals propagating through the optical fiber. In order to address SOP fluctuations, polarization-maintaining equipment is necessary. However, employing polarization-maintaining equipment throughout the transmission system may be extremely expensive or impractical. Thus, polarization controllers may be employed instead to alter the polarization state of the optical signal.
Polarization controllers receive an input SOP at a point along the transmission path and output an optical signal that will have a desired SOP at some later point along the transmission path. Typically, a polarization controller is composed of one or more “waveplates.”. As used herein, optical elements that exhibit birefringence are collectively referred to as waveplates. Birefringence is the separation of an incident light beam into a pair of diverging beams, known as “ordinary” and “extraordinary” beams. The velocities of the ordinary and extraordinary beams through the birefringent material vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. Waveplates can generate full, half and quarter-wave retardations when the phase difference equals whole, half and quarter wavelengths. Waveplates can also generate any arbitrary fractional-wave retardations. Various devices may be employed in a polarization controller, and modeled as a waveplate or combination of waveplates. Controller implementations can be classified into two types based on how the waveplate(s) operates: (1) devices based on control of waveplate birefringence while the effective waveplate axis is fixed, and (2) devices based on controlling the orientation of the waveplate about its axis while the birefringence is unchanged. Liquid crystal and fiber squeezer devices may fall into the first category while LiNbO3 waveguide devices and fiber loop devices may fall into the second category.
FIG. 1(a) illustrates an exemplary set of three (3) waveplates 10, 12 and 14 that can have their orientations (angles of rotation α, β, γ) changed while the birefringence is unchanged. As seen in the figure, an input SOP 16 can be modified by changing the orientation of the waveplate 10 to achieve a first intermediate SOP 18. The first intermediate SOP 18 can be modified by changing the orientation of the second waveplate 12 to achieve a second intermediate SOP 20. The second intermediate SOP 20 can be modified by changing the orientation of the third waveplate 14 to achieve an output SOP 22.
FIG. 2(b) illustrates a conventional single-channel polarization tracking receiver/filter implementing a polarization controller 200, which receives the multiplexed signal 116 from the wet plant 110. The polarization controller 200 operates on the multiplexed signal 116 and outputs a signal 206 to a polarization splitter 202. The polarization splitter 202 can separate a single channel (e.g., signal 208a) from the signal 206. The signal 208a is then provided to a receiver 204. A signal 208b is passed through an optical filter 210 and transmitted to the polarization controller 200 as a feedback signal. While only one polarization controller is shown in FIG. 2(b), it should be understood that separate polarization controllers 200 are employed for each channel.
One major concern in such transmission systems is the situation where the input SOP of an optical signal results in a feedback signal that is insensitive to the dithering or phase shifting of the rotational angle of the waveplate. This is known as “loss control.” The inventors of the instant application have identified loss control problems in both simulations and experiments. Others have also acknowledged loss control in the past, and have claimed the problem to be unavoidable. See, for example, Shieh et al., “Dynamic Eigenstates of Polarization,” IEEE Photonics Technology Letters, Vol. 13, No. 1, pp. 40 –42, January 2001, which is fully incorporated by reference herein. If loss control is not addressed in the polarization controller (either on the transmit side or on the receive side), it may not be possible to achieve a desired output SOP. The states that create loss control problems are known as “dead spots.” It is difficult to move away from a dead spot once it has been reached because conventional polarization controllers are not capable of making appropriate adjustments to the waveplates or other devices that they use. When a dead spot happens for a specific combination of waveplates in the polarization controller and a specific input SOP, small variations in the input SOP will require large changes to one or more of the waveplates to transform the input SOP to the desired output SOP. With a conventional dithering algorithm, however, the waveplates cannot be rotated by a large angle. Thus, in that situation, the output SOP may move away from a desired output SOP when the input SOP varies, resulting in a loss control situation. Dead spots can seriously degrade system performance and result in loss of received data due to co-channel interference and other problems.
Some conventional polarization controllers employ polarimeters. A polarization controller based on a polarimeter needs to know the birefringence transfer function from an input polarization state to the desired output polarization state through the controller device and transmission line (e.g., a single mode fiber). Determining the exact birefringence transfer function is not feasible in actual commercial systems. Thus, achieving a desired output polarization state is problematic. Other conventional polarization controllers have used a simple dithering algorithm based on the feedback signal from a polarization splitter to adjust the SOP. The dither algorithm is insensitive to, e.g., aging-induced drifting of controller device parameters such as DC bias voltage. For examples of polarization controllers employing the conventional dither algorithm, see “Analysis of a Reset-Free Polarization Controller for Fast Automatic Polarization Stabilization in Fiber-optic Transmission Systems,” Journal of Lightwave Technology, Vol. 12, No. 4, April 1994, and U.S. Pat. No. 5,212,743, both to Fred L. Heismann, which are fully incorporated by reference herein. In the Heismann references, a reset-free polarization controller is employed, which consists of several quarter-waveplates (QWP) and half-waveplates (HWP).
Specifically, a HWP is sandwiched between a pair of QWPs. The conventional approach is to dither the rotational angle of each waveplate as graphically illustrated in FIG. 3. The angle of the waveplate is dithered/adjusted by a small step-size (Δα, Δβ, or Δγ) in sequence. More specifically, the angle α of the first waveplate is dithered/adjusted (e.g., by mechanically rotating the waveplate) for a fixed time period, then the angle β of the second waveplate is dithered/adjusted for a fixed time period, and finally the angle γ of the third waveplate is dithered/adjusted for a fixed time period. Thus, each waveplate is independently dithered and adjusted for a fixed amount of time. Unfortunately, this approach may not have a sufficient control speed to handle fluctuations in the input SOP. This can result in a loss control problem.
There exist two situations that explain loss control and consequent reduction of the control speed. As used herein, control speed means that a polarization controller can track any random movement of the input SOP with a specific speed such that the desired output SOP is locked at some later point along the transmission path. In the first situation, there is little or no absolute response by dithering a waveplate (controlling the waveplate angle). In other words, the SOP may not change regardless of how much a particular waveplate is dithered. As an example, a HWP controller only transforms right (or left) circular polarization state at the input to left (or right) circular polarization state at the output independent of the rotation angle of the waveplate, which is the control parameter.
Second, there is only one direction of response by dithering any of the waveplates within the polarization controller. On a polarization plot using a Poincare chart, the SOP can be represented as a vector. Poincare charts are used to plot states of polarization in a three-dimensional format. Movement from one SOP to another SOP gives a trace on the Poincare chart. For an example, as shown in FIG. 1(b), a Poincare chart 30 traces the change in SOP from a left circular polarization state at the input of a polarization controller to a linear polarization state at the output, using a QWP-HWP polarization controller. The left circular polarization state is at point A on the sphere. Using the QWP and HWP waveplates alters the input SOP to a linear polarization state along the equatorial plane by means of a movement 32. Rotating/dithering either the QWP or the HWP can only generate a movement 34 along the circumference of the equatorial plane, and there is no movement along a longitudinal direction. Thus, a conventional polarization controller loses the tracking ability along the longitudinal direction.
The two situations discussed above are referred to herein as loss control (“LC”) effects. Because conventional polarization controller processes make changes to each waveplate for a fixed period of time, they are unable to sufficiently handle loss control problems. Thus, there is a need for new controller methods to address loss control problems.